I have a question about human perception. After reading the book "Sensory Perceptual Issues in Autism and Asperger Syndrome" and knowing about synesthesia (https://en.wikipedia.org/wiki/Synesthesia) something occurred to me.

If unique circuits of neurons are responsible for each of our senses, and the circuits can cross with each other (synesthesia), and given that autistic people can switch off a sense or may have problems filtering inputs, and all impulses are of one kind, how can the brain know where these inputs are coming from? For example if input is coming only from the eye, can a person think that it is coming from the ear? Can we see with our ears, or hear with our eyes, through some sort of circuit-crossing?

  • $\begingroup$ Thank you i highly aprreciate all feedbacks, because english is not my mother language and i am glad to learn more about it to improve my skills. $\endgroup$
    – L.Dodo
    Mar 1, 2018 at 11:57
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    $\begingroup$ You can write a thesis about that topic. You should rephrase it a bit, perhaps: At what stage in the circuits of neurons can sense perceptions of synesthesiacs become confused? What prevents ordinary people from seeing sounds and hearing what we see, if the circuits are eventually merged? Because subconcious perception is very fascinating that's an awesome topic to learn from, I donno if i will find a resonable response though, you need a pscyhologist/neurologist experience. $\endgroup$ Mar 7, 2018 at 15:49
  • $\begingroup$ I think you are confused about what is happening we can all filter out information (Sensory gating), we take in vastly more information than we pay attention to, autism reduces or removes the ability to filter out incoming signals. en.wikipedia.org/wiki/Sensory_gating $\endgroup$
    – John
    Mar 7, 2018 at 17:51
  • $\begingroup$ Good question; I tried to rewrite your title to be more of a question format rather than a topic heading - feel free to edit it if this wasn't quite the question you meant to ask. $\endgroup$
    – Bryan Krause
    Mar 8, 2018 at 17:42
  • $\begingroup$ i think you didnt change the meaning. I even love that format !! :D thank you very much $\endgroup$
    – L.Dodo
    Mar 8, 2018 at 18:16

1 Answer 1


Short Answer

Synesthesia happens at a point during processing where we are not dealing with "raw visual input" or "raw auditory input" anymore, but already with more abstract constructs such as "colors" or "sounds".

Full Answer

Your question seems to consider sensory perception as a matter of "inputs" and the brain detecting those inputs, such that for example the only difference, from the brain's point of view, between visual inputs and auditory inputs is that one comes from the ear and the other from the eye.

A more accurate way of seeing it is that our sensory perceptions are really the brain constructing a coherent reality from the information it got from our senses. And I do mean "information" in the most abstract, computer-sciency sense possible. It's not like our eye is a photographic plate that contains "what we see", and the brain just has to reconstruct that. The processing of visual information, by which I mean picking out various features from the light received and deriving meaning from them, starts right in the retina. Some examples from the "14.5 Visual Processing in the Retina" section in this page:


On bipolar cells:

The two bipolar cell types have different functional properties.

  • The off bipolar cells function to detect dark objects in a lighter background.

  • The on bipolar cells function to detect light objects in a darker background.

On horizontal cells:

The surround effect, produced by the horizontal cells, enhances brightness contrasts to produce sharper images, to make an object appear brighter or darker depending on the background and to maintain these contrasts under different illumination levels.

On the retinal ganglion cells:

The retinal ganglion cells provide information important for detecting the shape and movement of objects.

Type P retinal ganglion cells are color-sensitive object detectors.

Type M retinal ganglion cells are color-insensitive motion detectors.

On amacrine cells:

There are 20 or more types of amacrine cells based on their morphology and neurochemistry. The roles of three types have been identified. One type

  • is responsible for producing the movement sensitive (rapidly adapting) response of the Type M ganglion cells.

  • enhances the center-surround effect in ganglion cell receptive fields.

  • connects rod bipolar cells to cone bipolar cells, thus allowing ganglion cells to respond to the entire range of light levels, from scotopic to photopic.

Similarly in our ears, the cilia cells that vibrate in response to sound are frequency-dependent, and it is information about those frequencies that gets transmitted to the rest of the brain for processing. In other words the very organ that detects sound also performs a kind of Fourier transform on the sound waves it detects before anything else.

See for example:


This parsing, combining and interpreting of information continues throughout the perceptual circuits of the brain, and not in a straightforward way either - it is hypothesized that there are two independent or semi-independent processing streams for visual information, the ventral and dorsal streams, one which is for identifying objects and the other for guiding actions:


There is also information flowing the other way, where more "advanced" brain regions draw conclusions about what's actually there and feed those conclusions back into the more "basic perception" parts to ensure that that's what we see. I.e. perception has both "feedforward" and "feedback" processes, or "bottom-up" and "top-down".

All of this is to say, there is absolutely no way for the brain to confuse information from the retina and information from the ear; the early processing in both of those is too tightly linked to the organ itself, and perception as a whole is inextricably linked to cognition. It's not like a mouse and keyboard that you can plug into different sockets; the retina technically is part of the brain in the first place!

This is not to say visual and auditory senses cannot be confused, obviously synesthesia happens, but it likely happens at a point in the processing where we're not dealing with "raw visual input" or "raw auditory input" anymore, but already more abstract constructs such as "colors" or "sounds".

For example this paper (gotten from the Wikipedia page https://en.wikipedia.org/wiki/Neural_basis_of_synesthesia) :


Hypothesizes that the synaesthetic association of letters with colors is caused by cross-wiring between the area of the visual cortex that processes letters and the one that processes colors.

This paper from the same source:


Finds that color-word synaesthetes had, when told words the associated with colors, activation in the language and "advanced" visual processing areas but not in more basic visual processing areas.

Similarly for autism, given that sensory perception happens at every level in the brain, including conscious thought, those perceptions can be blocked or filtered (or not) by the brain at any stage. No need to assume it is at the "raw input" stage.

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    $\begingroup$ Nice answer, I hope you don't mind my edit to bring the punchline up to the top. $\endgroup$
    – Bryan Krause
    Mar 8, 2018 at 17:39

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